linux-kernel - Re: [PATCH 3/3] [RFC] nmi_watchdog: config option to enable new nmi

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Date:	Fri, 29 Jan 2010 09:12:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, gorcunov@...il.com,
	aris@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] [RFC] nmi_watchdog: config option to enable new
 nmi_watchdog


* Don Zickus <dzickus@...hat.com> wrote:

> On Thu, Jan 28, 2010 at 03:54:54PM +0100, Peter Zijlstra wrote:
> > On Wed, 2010-01-27 at 15:03 -0500, Don Zickus wrote:
> > > These are the bits that enable the new nmi_watchdog and safely isolate the
> > > old nmi_watchdog.  Only one or the other can run, not both at the same
> > > time.
> > 
> > perf disables the lapic watchdog when it wants the pmu, so there
> > shouldn't be a problem having both built in.
> 
> Yes it does disable but does not prevent nmi_watchdog_tick from running nor 
> the /proc interface from being loaded.  So perhaps my description isn't very 
> good.  The idea with the new watchdog was to re-use some of the bits of the 
> old one, but having them both compiled in seemed to stomp on each other.  
> That is what I was trying to prevent.
> 
> I can certainly change the behaviour, just makes the code a little more 
> messy I think.

I think that's a good idea - and i think we want to be bold and just have the 
new code run seemlessly. (and fix bugs, if any.)

In fact we want to be even bolder: how about enabling the NMI watchdog by 
default again?

The problem with the old one was its fragility - but now if we have a PMU 
driver active and perf events enabled we might as well use your brand new NMI 
watchdog code as a testing facility as well: if there's _any_ problem with 
NMIs then regular 'perf' use would trigger it too - except that not all people 
run perf while an always-enabled NMI watchdog would.

And it would detect hard hangs too.

What do you think?

	Ingo
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